1. Technical Field
This invention relates to complex oxide fine particles represented by the general formula: ABO3 wherein A is at least two elements selected from among alkaline earth metal elements and rare earth elements and B is at least one metal selected from among Ti, Mn, Nb and Zr; a particulate mixture of the complex oxide fine particles and nickel particles; and a conductive paste using the mixture.
2. Background Art
In unison with the recent demand to reduce the size of portable equipment and digital home appliances, studies have been made on multilayer ceramic capacitors to reduce their size and to increase their capacitance. For the size reduction and capacitance increase of multilayer ceramic capacitors, it is most effective to establish a multilayer structure by reducing the thickness of dielectric layers and internal electrode layers. For reducing the thickness of internal electrode layers, metal particles in a conductive paste from which the internal electrode layers are formed must be made finer. Among the currently available internal electrode layers, the thinnest layers are of the order of 1 micron (μm), and made of a conductive paste containing metal particles which are composed of a single metal such as nickel, silver, palladium or the like and have an average particle diameter of about 0.2 to 0.5 micron. Engineers are making efforts to develop the technology capable of forming internal electrode layers as thin as about 0.5 micron. To this end, it is believed that the metal particles in conductive paste must have an average particle diameter of 0.2 micron or less.
However, as metal particles become finer, their incipient shrinkage temperature becomes lower and their shrinkage characteristics become greater. It is considered problematic that when multilayer ceramic capacitors are sintered, cracks and other defects develop due to differential thermal shrinkage, especially a difference in incipient shrinkage temperature, between internal electrode layers composed mainly of nickel metal and dielectric layers composed mainly of ceramics such as BaTiO3. A number of proposals have been made to overcome this problem.
More particularly, finer metal particles have a larger surface area and a lower incipient shrinkage temperature. For example, internal electrode layers composed mainly of nickel metal start shrinkage from a temperature of about 600° C. On the other hand, dielectric layers composed mainly of ceramics such as BaTiO3 start shrinkage from a temperature of about 900° C. Then at the temperature of about 600° C. at which internal electrode layers start shrinkage, the binder in dielectric layers has already been burned out so that the dielectric layers are weak (low strength) and susceptible to defects such as cracks by the shrinkage of internal electrode layers.
One known attempt to elevate the incipient shrinkage temperature or to restrain the shrinkage factor of internal electrode layers is by adding to nickel metal particles an oxide having little impact on dielectric properties. For instance, it was proposed to add magnesium and/or calcium (see Japanese Patent No. 2,945,644) or rare earth oxides such as Y2O3 or BaTiO3 (see Japanese Patent No. 2,992,270). Both the proposals are successful in elevating the incipient shrinkage temperature of internal electrode layers, but to a less extent. The same assignee as the present invention proposed particles in which core particles made of a high melting point nickel alloy are covered with an oxide (JP-A 2002-302701).